The ITU (International Telecommunication Union) has adopted plural wireless communication methods called the 3rd generation as IMT-2000 for mobile wireless communication method typically used in the field of cellular phones. In Japan W-CDMA (Wideband Code Division Multiple Access) method as one of them is commercially available from 2001.
W-CDMA is made to obtain a communication speed of the maximum 2 Mbps (bit per second) per mobile station. The 3GPP (3rd Generation Partnership Project) as one of standardization groups has determined the specification of the first edition as Release 99 version (Release 1999) which was summarized at 1999.
FIG. 1 is a general schematic diagram of a conventional communication system. In FIG. 1, reference number 1 designates a base station, and 2 denotes a mobile station performing a wireless communication with the base station 1. Reference number 3 indicates a downlink for use in data transmission from the base station 1 to the mobile station 2, and 4 indicates an uplink for use in data transmission from the mobile station 2 to the base station 1.
FIG. 2 is a diagram showing an internal configuration of the mobile station. 2. In FIG. 2, reference number 11 designates a distributor for distributing data DPDCH of a dedicated data channel (Dedicated Physical Data Channel) in parallel and outputting obtained data DPDCH1–DPDCH6 of plural data channels. Reference number 12 denotes a spreader for performing a spread spectrum process for data DPDCH1–DPDCH6 output from the distributor 11 and control data DPCCH of a control channel (Dedicated Physical Control Channel). The spreader 12 multiplies the data DPDCH1–DPDCH6 and the control data DPCCH by spreading codes for channel separation.
Reference number 13 indicates a scrambler for generating a complex signal (I signal: Inphase signal, Q signal: Quadrature signal) by performing IQ multiplexing for output signals from the spreader 12. Reference number 14 denotes a modulator for generating a modulated signal by performing orthogonal modulation of a complex signal (I signal and Q signal) generated at the scrambler 13. Reference number 15 indicates a frequency converter for converting in frequency the modulated signal generated at the modulator 14 to a radio frequency signal. Reference number 16 designates an antenna for transmitting the radio frequency signal output from the frequency converter 15.
FIG. 3 is a diagram showing an internal configuration of the spreader 12 and the scrambler 13. In FIG. 13, reference numbers 21 to 26 indicate multipliers for multiplying the data DPDCH1–DPDCH6 output from the distributor 11 by spreading codes Cd,1 to Cd,6 for use in channel separation. Reference numbers 27 designates a multiplier for multiplying the control data DPCCH of the control channel by a spreading code Cc for use in channel separation. Reference number 31 to 36 denote multipliers for multiplying the output signals from the multipliers 21 to 26 by an amplitude coefficient βd for the data DPDCH. Reference number 37 designates a multiplier for multiplying the output signal from the multiplier 27 by an amplitude coefficient βc for the control data DPCCH.
Reference number 38 denotes an adder for adding the output signals from the multipliers 31 to 33, and 39 denotes an adder for adding the output signals from the multipliers 34 to 37, Reference number 40 denotes a multiplier for multiplying the output signal from the adder 39 by imaginary number “j”, 41 indicates adder for adding the output signals from the adder 38 and the multiplier 40. Reference number 42 designates a multiplier for multiplying the output signal from the adder 41 by an identification code Sdpch,n for a cellular station in order to generate the complex signal (I signal and Q signal), and then outputting the generated complex signal.
Next, a description will be given of the operation of the conventional communication system in which data are transmitted from the mobile station 2 to the base station 1.
When transmitting data to the base station 1, as shown in FIG. 1, the mobile station 2 uses the uplink 4 for the transmission data. In W-CDMA standard, when using the uplink 4, the mobile station 2 can use maximum six channels for the transmission data according to a communication speed required in communication service.
In the following explanation, data on six data channels and control data for one control channel are transmitted for brief explanation.
First, the distributor 11 in the mobile station 2 distributes the data DPDCH of the dedicated data channel in parallel and outputs the data DPDCH1–DPDCH6 for the plural data channels.
When the distributor 11 outputs the data DPDCH1–DPDCH6 for the plural data channels, the multipliers 21–26 in the spreader 12 multiply these data DPDCH1–DPDCH6 with the spreading codes Cd,1–Cd,6 for channel separation. The multiplier 27 in the spreader 12 multiplies the control data DPCCH for the control channel by the spreading code Cc for channel separation.
The scrambler 13 performs IQ multiplexing for the output signal from the spreader 12 in order to generate the complex signal (I signal and Q signal).
That is, the multipliers 31–36 in the scrambler 13 multiply the output signals from the multipliers 21–26 in the spreader 12 by the amplitude coefficient βd. The multiplier 37 in the scrambler 13 multiplies the output signal from the multiplier 27 by the amplitude coefficient βc for the control data DPCCH.
FIG. 4 is a diagram showing a table of possible values of the amplitude coefficients βc and βd.
The amplitude coefficients βd and βc are coefficients for use in the determination of a power ratio between the data DPDCH1–DPDCH6 and the control data DPCCH, which have been defined in TS25.213 v3.6.0(2001–06) Release 1999 in 3GPP standard. Right side in this table shows the possible values of the amplitude coefficients βc and βd.
The adder 38 in the scrambler 13 adds the output signals from the multipliers 31–33 and the adder 39 in the scrambler 13 adds the output signals from the multipliers 34–37.
The multiplier 40 in the scrambler 13 multiplies the output signal from the adder 39 by imaginary number “j” so as to assign the output signal from the adder 39 to Q axis.
The data DPDCH1, DPDCH3, and DPDCH5 are assigned on I axis and the data DPDCH2, DPDCH4, and DPDCH6 are assigned on Q axis. TS25.213 in 3GPP standard defines how to assign data channels on I axis/Q axis.
Next, the adder 41 in the scrambler 13 adds the output signals from the adder 38 and the multiplier 40. The multiplier 42 in the scrambler 13 multiplies the output signal from the adder 41 by an identification code Sdpch,n to be used to identify a dedicated mobile station, and then outputs the complex signal (I signal and Q signal).
When the scrambler 13 generates the complex signal (I signal and Q signal) in such a manner described above, the modulator 14 performs the orthogonal modulation for the complex signal (I signal and Q signal) so as to generate the modulated signal.
When the modulator 14 generates the modulated signal, the frequency converter 15 converts this modulated signal in frequency, generates the radio frequency signal, and amplifies and outputs the generated one to the antenna 16. Through the antenna 16 the radio frequency signal is transmitted to the base station 1.
When receiving the radio frequency signal transmitted from the mobile station 2, the base station 1 performs inverse processes to the processes in the mobile station 1 in order to obtain the necessary data.
The above conventional case has explained the case to set the six data channels. When the set number of the data channels is not more than 5, no process for unnecessary data channel is performed because the data are assigned on I axis and Q axis in the order of increasing data number, for example, the data DPDCH1 is firstly assigned and the data DPDCH2 is then assigned. The set number of the data channels is determined based on the communication service and the communication speed.
FIG. 5 is a diagram showing a complex plane of only one data channel.
In this case, the data DPDCH1 for the data channel is assigned on I axis and the control data DPCCH for the control channel is assigned on Q axis. Because the data DPDCH1 and the control data DPCCH are orthogonal to each other, the base station 1 can separate the received data in channel and then demodulate the separated data.
It is possible to perform the same operation for the case where the set number of the data channels is 2, 3, 4, 5, or 6. In this case, the channel component in the same axis can be separated using the spreading code for channel separation.
The above conventional example has described the case to set the downlinks 3 and the uplink 4 between the base station 1 and the mobile station 2. In order to achieve a further high speed data communication in the downlink from the base station 1 to the mobile station 2, HSDPA (High Speed Downlink Packet Access) has been proposed and examined (see TR25.858 v1.0.0 (2001–06) “High Speed Downlink Packet Access: Physical Layer Aspects (Release 5)”.
FIG. 6 shows HSDPA in which a new downlink 5 is added in addition to the downlink 3 in the conventional case.
In the addition of the new downlink 5, it has been examined that the mobile station 2 transmits a response data (ACK/NACK) and the like to the high speed packet data in the downlink to the base station 1. However, as shown in FIG. 6 in which the response data (ACK/NACK) is transmitted through the exclusive control channel (as the uplink channel 6). Through the exclusive control channel the response data are separated and identified using the spreading code for channel separation, like the same manner for the conventional control channel, and then added and multiplexed in the conventional uplink 4. TR25.858 defines to describe “additional DPCCH” as the exclusive control channel.
Because the conventional communication system has the configuration described above, it is necessary to assign the additional exclusive control channel on I axis and Q axis. This causes a drawback where a distortion is generated at the built-in orthogonal modulator (or orthogonal modulator and amplifier) in the modulator 14 in the mobile station 2 because nonlinear section of input/output characteristic must be used, when the peak power of I axis or Q axis is increased by assigning the exclusive control channel to I axis or Q axis, for example.
When the balance between the signal powers of I axis and Q axis is decayed, the peak power of the modulated signal output from the modulator 14 after the orthogonal modulation is greater than the peak power of the modulated signal of the case where the signal powers of I axis and Q axis are in balance. For example, in case an amplifier incorporated in the frequency converter 15 in the mobile station 2 amplifies the radio frequency signal, a distortion occurs because the amplifier uses in amplification a non-linear part of the input/output characteristic thereof. When the non-linear component in the distortion generated in the amplifier is output, this non-linear component and the signal component of the frequency band adjacent to this linear component interfere to each other. The reception of the adjacent frequency band is thereby disturbed by jamming.
The present invention is made to overcome the above drawbacks. It is an object of the present invention is to provide a mobile station, a base station, a communication system, and a communication method which are capable of suppressing the generation of a distortion in amplifiers and thereby to suppress the occurrence of jamming in the adjacent frequency band.